Cardiovascular diseases are an important cause of mortality and morbidity. Amongst all age groups considered, IHD is the most common cause of death not only in men but also in women. Coronary atherosclerosis is a chronic progressing process, associated with angina type symptoms and frequently result in Acute Myocardial Infarction (AMI). The diagnosis is achieved with a combination of patient physical examination, ECG since 1950's molecular markers play the most important role in the differential diagnosis of AMI from other conditions with similar symptoms. Early diagnosis is mandatory of the establishment of early treatment (including blood diluting agents, thrombolysis, catheterization and surgery).
Early molecular markers for AMI were SGOT and LDH were proved to be of very low specificity and are hardly being used at present. These markers were replaced by CPK, and later on by the heart specific CPK-MB variant. Its specificity is better than for SGOT and LDH, it is still limited both in specificity and sensitivity which reach only 67% when used together with electrocardiogram. In addition, cardiac surgery, myocarditis, and electrical cardioversion often result in elevated serum levels of the CPK-MB isoenzyme. Small infarct with minor myocardial cell necrosis often do not increase serum CPK-MB to a detected level. Myoglobin is another heart damage low molecular (17 kD) protein but is even less specific to heart muscle compared with CPK-MB. Its advantage over CPK-MB is a rapid rise from the onset of symptoms—usually between 3-6 hours. It is considered one of the earliest indicators (together with H-FABP) but it lacks specificity due to significant expression in skeletal muscle—its concentration is approximately two-fold lower in cardiac than skeletal muscle and the leads to seriously diminished specificity.
Cardiac troponins are currently the routine serum cardiac markers used for the diagnosis of AMI. Troponin-I and Troponin-T have amino acid sequences different from those of the skeletal muscle called cTnT and cTnI (cardiac Troponin-T and I recpectively). Cardiac troponins are not found in the serum of healthy individuals and rise to up to 20 times above a predefined cut-off level, therefore are very useful and sensitive in the detection of cardiac damage. They are capable of detecting very small cardiac damage—micro-infarction, it is associated with a very adverse longer term prognosis. Cardiac troponin's sensitivity is considerably higher than CPK-MB but they suffer from a few disadvantages: 1. They are not early markers—cTnI and cTnT reach peak serum value in about 12 and 48 hours respectively after symptoms onset. 2. Levels of cTnI and cTnT remain elevated for up to 10 days and 14 days respectively after AMI, therefore cannot be used for the detection of re-infarction. 3. Other heart diseases such as Congestive Heart Failure and Myocarditis can increase troponins concentrations in the serum. The lack of specificity for AMI is an advantage when there are other supporting clinical evidence directing the doctor towards another diagnosis. Troponins might have a diagnostic value in assessing myocardial damage after coronary artery perfusion, monitoring progression and prognosis of unstable angina, in the detection and prognosis of cardiac contusion after blunt trauma, detecting myocarditis.
The heart specific variant H-FABP (Heart Fatty Acid binding protein) is a low molecular protein (15 Kd) soluble non-enzyme protein. H-FABP concentration in the heart muscle is greater than that in skeletal muscle, and its normal baseline concentration is several fold lower than myoglobin. In addition, it reaches peak value in the urine and blood early, within 2-3 hours from AMI. Within a period of 30-210 minutes after symptoms started, H-FABP has higher sensitivity—up to 80%—when compared with other cardiac markers (CPK-MB and the troponins sensitivity were reported to be 64% in the first 6 hours after AMI). Yet, H-FABP still misses every 5th patient in this time scale. H-FABP has other limitations as well, including 1. rising in the plasma after exercise 2. released from muscle in skeletal damage during the course of AMI (like from intramuscular injections) 3. reduced clearance in renal failure situations.
The search for novel cardiac damage markers is ongoing. Other proteins are under trials for that purpose including glycogen phosphorylase BB, HIF and VEGF 21.